1. Field of the Invention
The present invention relates to twinaxial cables, and more specifically, the present invention relates to twinaxial cable ribbon.
2. Description of the Related Art
Coaxial cables include a center conductor surrounded by an insulation layer and a conductive shield covering the insulation layer. Typically, a protective layer covers the conductive shield.
Twinaxial cables are similar to coaxial cables, but contain two center conductors instead of one. The two center conductors of a twinaxial cable are each surrounded by an insulation layer. Typically, the insulation layers for each of the two center conductors of twinaxial cables are separately formed. A conductive shield covers the insulation layers.
Twinaxial cables are often used to provide high-speed differential signaling in electrical devices. For example, in differential signaling, complementary signals may be sent on respective center conductors of a pair of center conductors.
Typical applications for twinaxial cables include those applications in which a differentially controlled impedance is required to transfer data at a high rate. These typical applications require a high-performance electrical path across longer distances and/or a system architecture with inherent signal routing or mechanical packaging problems that can only be overcome by thin, fine-pitch, flexible, high-performance twinaxial cable. Typical applications include, for example, datacenter servers and other modules, supercomputing, medical imaging, and military applications.
Twinaxial cables are conventionally manufactured as a ribbon that includes two or more pairs of center conductors for carrying two or more differential signals. Thus, in a twinaxial cable ribbon, multiple twinaxial cables are provided. Each of the twinaxial cables includes two center conductors, each of the two center conductors is surrounded by a insulation layer, and a conductive shield covering the insulation layer.
Twinaxial cable ribbons are manufactured in several ways. For example, one known method, shown in FIG. 7, laminates an adhesive metal tape 1080 across several twinaxial cables. The thermal set adhesive used in the adhesive metal tape 1080 increases the resistance of this conventional laminated twinaxial cable ribbon. Further, a space or gap is typically provided between each of the twinaxial cables in the laminated twinaxial cable ribbon 1000 such that the center-to-center distance of two adjacent twinaxial cables is about 3 mm or more. This space is required to prevent crosstalk between adjacent twinaxial cables. Crosstalk between adjacent twinaxial cable pairs can easily occur because a conductive shield is not provided around the insulation layers 1040, 1045 around each of the center conductors 1050, 1060.
Another problem with laminating the adhesive metal tape 1080 is the creation a tangent shield gap 1070 between the adhesive metal tape 1080 and the center conductors 1050, 1060. The tangent shield gap 1070 causes variations in the electrical properties of the twinaxial cables so that the laminated twinaxial cable ribbon 1000 cannot be used as a high performance cable because high performance cables require consistent electrical properties.
Another known method of manufacturing a ribbon uses a solvent to partially melt an outer layer of PVC and combine adjacent twinaxial cables. This method requires that each twinaxial cable of the ribbon have an outer PVC layer. A problem with this arrangement is that the outer PVC layer of each of the twinaxial cables increases the pitch between adjacent twinaxial cables. Other problems are that this method requires the use of expensive materials, such as Teflon®, and that it requires multiple extrusions steps to prepare each of the twinaxial cables.
Known twinaxial cable ribbon may also include one or more drain wires, each of the one or more drain wires being separated from the twinaxial cables by a certain distance. The one or more drain wires may be electrically connected to each of the conductive shields of the twinaxial cables in order to provide a convenient ground connection. As one example, described below, hot bar soldering may be used to electrically connect the one or more drain wires to each of the conductive shields.
FIG. 7 shows a conventional laminated twinaxial cable ribbon 1000. A first center conductor 1050 is surrounded by a first layer of insulation 1040. A second center conductor 1060 is surrounded by a second layer of insulation 1045.
As shown in FIG. 7, twinaxial cables of the conventional laminated twinaxial cable ribbon 1000 include a separate layer of insulation 1040, 1045 for each of the respective center conductors 1050, 1060. Furthermore, as shown in FIGS. 7 and 8, adjacent pairs of center conductors 1050, 1060 are spaced apart by about 3 mm or more.
A drain wire 1020 is separated by a certain distance from the closest pair of center conductors 1050, 1060. Accordingly, the drain wire 1020 is not in direct contact with the conductive shields of the first center conductor 1050 and the second center conductor 1060.
As shown in FIG. 8, one problem with conventional twinaxial cable ribbons is that a connection between the conventional twinaxial cable ribbon 1000 and a circuit board 1200 requires a large footprint 1210 on the circuit board 1200, due to the spacings between each of the twinaxial cables and the spacings between the one or more drain wires and the twinaxial cables. Typically, a circuit board has conductive pads aligned with the conductors and the drain wire(s) of a corresponding cable. The footprint 1210 on the circuit board 1200 for connecting to the conventional twinaxial cable ribbon 1000 refers to a surface area on the circuit board 1200 that is occupied for the layout of the electrical connections to the cable 1000. Accordingly, the spacings between the twinaxial cables and between the drain wires and the twinaxial cables result in a relatively low-density cable that requires a large footprint, which may increase the cost of a system that uses the conventional twinaxial cable ribbon.
Another problem with conventional twinaxial cables is a variation in the electrical properties among each of the layers of insulation, since each of the center conductors has a separate insulating dielectric layer. Since separately formed insulators do not have identical electrical properties, performance is degraded as compared to, for example, a pair of center conductors that share a single insulating layer. Separately formed insulating layers also require an additional assembly step in the manufacturing of conventional twinaxial cables, since multiple extrusion steps are required to form the insulation layers on each of the center conductors. Accordingly, the performance of the twinaxial cable may be further degraded due to stresses on the twinaxial cables during this extra manufacturing process.
An additional problem with conventional twinaxial cables is a non-constant distance between the center conductors of a differential pair due to shifting of the separately formed insulators during twisting, bending, stretching, or similar stresses of which twinaxial cables are often subjected. Furthermore, any shifting of the relative positions of each of the center conductors may cause variation in the electrical properties of the twinaxial cables.
A further problem with conventional twinaxial cable ribbons is creating the connections between the conductive shields and the center conductors of the twinaxial cables and a circuit board. For example, it is known to create these connections using hand soldering and hot bar soldering. Hand soldering is time consuming and expensive. Hot bar soldering has been used as a convenient process to obtain a common electrical connection between the conductive shields and the center conductors of the twinaxial cables of conventional twinaxial cable ribbons and a circuit board. A problem with conventional hot bar soldering is that it requires two hot bar solder steps: once to solder the conductive shields and once to solder the center conductors. In conventional twinaxial cable ribbons that use drain wires, it is possible to connect the drain wire to the conductive shields of the twinaxial cables using hot bar soldering.
In a typical hot bar soldering process, each of two elements that are to be connected are separately coated with a layer of solder. The two elements are then pressed together and then heated to melt, or reflow, both layers of solder. After heating, the elements are cooled while still being pressed together, and an electro-mechanical connection is obtained when the solder solidifies.
However, hot bar soldering a served or braided conductive shield requires the use of materials that can withstand the high temperatures applied during the heating step of the hot bar soldering process. Accordingly, the insulation layers of conventional twinaxial cables must be formed of expensive dielectric materials that can withstand the high temperatures used in hot bar soldering. A common dielectric material used for insulation layers in conventional twinaxial cables is fluorinated ethylene propylene (FEP). Lower cost dielectric materials, such as polypropylene, are unable to withstand the temperatures applied in hot bar soldering a served or braided conductive shield.